Method for making an insulator



May 6, 1969 R. F. REA 3,442,693

METHOD FOR MAKING AN INSULATOR Filed April 15. 1966 INVENTOR.

HUBBRT f. REA BY QMCZM ATTYS.

United States Patent US. Cl. 117123 6 Claims This invention relates to a method for making an insulator, and, more particularly, to a method for producing a ceramic body which is particularly useful for separating a center electrode and a metal shell in a spark igniter, and insulating the two from one another electrically, but having a spark gap surface along which a spark travels when the igniter assembly is in service, the spark gap surface being impregnated with a plurality of minute, separate bodies of a noble metal.

Spark igniter assemblies, as have previously been known, differ significantly from conventional spark plugs, as well as require a different ignition system. In a spark plug, the firing end of a center electrode and a ground electrode usually are separated by a relatively small air gap; when a high voltage difference is established between the center electrode and the ground electrode, the air gap is ionized and a spark discharge occurs therethrough between the ground and the center electrodes. In an igniter assembly, on the other hand, a surface of a ceramic insulator, separating the center electrode from the metal shell, which is usually the exterior part of an igniter, is disposed adjacent the path between the center electrode and the ground electrode, so that the spark occurs along such surface. An igniter is sparked by an ignition system which includes a capacitor. The capacitor is charged before a spark occurs, and discharged during sparking; as a consequence, a large part of the electrical energy stored by the capacitor is released, and causes a high energy spark.

Spark igniter assemblies are, generally, of one or the other of two types, so-called high voltage igniters and so-called low voltage igniters. conventionally, the spark gap surface of a high voltage igniter is merely a part of the insulator, with no treatment whatsoever, and has a resistance (measured by a 500 volt megger) of 200+ megohms at room temperature, although a somewhat lower resistance at its normal operating temperature. To be satisfactory for general use, the insulator must have such electrical resistance properties that a one centimeter cube thereof, at 1350 F., has a volume of resistivity of at least one megohm.

A low voltage igniter assembly usually has an electrically semiconducting spark gap surface, which may be merely a surface coating on a conventional insulating body, or which may be a solid button of an electrically conducting material fastened to an end of a conventional insulator. T-he semiconducting surface usually has a room temperature resistance, measured by a 500 volt megger, of 100,000 ohms or less.

The instant invention is concerned with an insulator for a high voltage spark igniter assembly having a high resistance spark gap surface impregnated with minute bodies of a noble metal. Previous attempts have been made to provide such surfaces by impregnating the insulator, after calcining but before firing, with a noble metal solution. The prior concentrations, however, were insufficient to achieve. a significant decrease in the onset voltage requirement to produce a desired spark and did not appreciably alter the initial electrical resistance of the ceramic, as measured by a 500 volt megger. The invention is based upon the discovery that the spark gap surface of the insulator can be impregnated with a higher concentration of minute bodies of a noble metal than was heretofore possible to provide a substantial reduction in the onset voltage. This is achieved by providing the interior of the insulator with hydrophobic characteristics in order to prevent the noble metal solution from migrating into the interior of the insulator to an extent such that the concentration of the metal near the surface is decreased. By preventing such migration, the metal can be retained at the surface under much higher concentrations than heretofore possible, and less noble metal is deposited deeper below the surface where it is wasted.

It is, therefore, an object of the instant invention to provide a method for producing an insulating ceramic body for use in a spark igniter assembly.

Other objects and advantages will be apparent from the description which follows, reference being made to the drawing, which is a view, partially in section, of a typical spark igniter assembly which includes an insulator produced according to the invention.

The invention relates to an improvement in an electrically-insulating ceramic body for a spark igniter assembly and in a method for producing same. Such method includes the steps of forming ceramic batch material into a required shape, including a gap surface along which a spark travels when the insulating body is used in an igniter assembly, and firing the shape to vitrification. The improvement according to the invention involves adding to the ceramic batch material a hydrophobic material. Stearic acid has been used with good results; other suitable hydrophobic materials include stearates of aluminum, calcium, sodium, and zinc, as well as many waxes. The batch material is then formed to a predetermined shape and fired briefly at a relatively low temperature, the time and temperature being snfiicient to cause the hydrophobic material near the surface of the green insulator to be eliminated by vaporizing or burning. The hydrophobic material preferably is removed to a depth of 0.002 inch to 0.005 inch. A lesser depth results in too thin a layer of noble metal being subsequently for-med to provide a significant effect while a greater depth results in an excessive waste of the noble metal. At this point, the insulator is impregnated with an aqueous solution of a noble metal and dried. The impregnation and drying can be repeated for greater concentrations of the metal near the surface of the insulator but, in all cases, the solution is prevented from impregnating the insulator beyond the remaining hydrophobic material so that the noble metal is concentrated at the surface to the maximum extent, rather than progressing further into the insulator and reducing the concentration at the surface, as has heretofore occurred. Less noble metal is thereby required for a given concentration so that the overall cost of the process and the insulator is decreased. The impregnated insulator is then fired to vitrification to complete the process. The noble metal also can be added to the ceramic body after shaping and calcining. To accomplish this, after calcining, the body is impregnated with the wax and subsequently fired to remove the wax from the surface. The body is then impregnated with the noble metal solution.

Optimum results have been achieved where the noble metal solution with which the calcined insulator is impregnated has been platinum. However, significant improvement can also be achieved using an aqueous solution comprising iridium, ruthenium, gold, osmium or rhodium. The solution should contain from 0.3 gram to 0.9 gram of the noble metal per milliliter; optimum results have been achieved when the solution contained about 0.58;0L012 gram/milliliter.

Optimum results in accordance with the invention have also been achieved when the ceramic batch materials have been high in alumina, e.g., have contained at least percent thereof. Such compositions are well known as conventional high alumina spark plug insulator batches, and may mature after firing to Cone 16', about 2700 F., or to Gone 31, about 3060 F., in most instances depending upon the specific proportions of various fiuxing ingredients contained therein.

Referring now in detail to the drawing, a typical spark igniter assembly which includes an insulator produced according to the invention is indicated generally at 1. The assembly 1 comprises a metal shell 2 which is threaded at 3 for engagement in an internal combustion engine, most frequently a jet engine, and also at 4 for engagement with an ignition harness. A ceramic insulator 5 is seated within the shell 2, adjacent the lower extremity thereof, and an upper ceramic insulating member 6 is disposed above the member 5, in the specific assembly shown. Gas-tight seals 7 and 8 are provided between the insulating member 5 and the shell 2 and between the upper insulating member 6 and the shell 2, respectively. A center electrode assembly indicated generally at 9 is seated in central bores which extend through the insulating members 5 and 6. The electrode assembly 9 comprises a center electrode 10 having a firing end 11, and a terminal 12 which is electrically connected to the electrode 10. A gastight seal 13 is also provided to prevent leakage of gas from an associated engine through the central bores of the insulating members 5 and 6.

The lower extremity of the metal shell 2 of the igniter assembly 1 has an inwardly directed flange 14 which includes a surface 15 that is, in essence, a ground electrode for the assembly. A frusto-conical surface 16 of the lower insulating member 5 is disposed adjacent the spark path between the firing tip 11 of the center electrode 10 and the surface 15. The surface 16 or an equivalent surface is impregnated with discrete, minute bodies of a noble metal such as platinum, rhodium, iridium, ruthenium, gold, or osmium by the method of the instant invention.

The following example is presented solely for the purpose of further illustrating and disclosing the invention, and is in no way to be construed as a limitation thereof.

EXAMPLE Conventional high alumina spark plug batch material was mixed, containing about 92 percent of alumina and minor amounts of whiting, talc, and bentonite. To this batch of material was added stearic acid in an amount of two percent by weight. The batch material was then shaped under pressure into green insulators of the configuration of the insulator 5 in the drawing. The insulators were then placed in an oven maintained at 1500 F. for three minutes, which was sufficient to cause the stearic acid near the surfaces of the insulators to be removed and leave the outer parts of the insulators relatively porous. The time was varied for different insulators from two-and-one-half to four minutes which changed the depth of the permeable part from approximately 0.002 inch to approximately 0.004 inch, although depths to 0.025 inch are satisfactory and not uncommon. The spark gap surfaces 16 of the insulators, as well as adjacent surfaces, were then impregnated with a water solution of chloroplatinic acid containing 0.58 gram of platinum per milliliter. The impregnation in this instance was accomplished by brushing. The insulators were then dried, reimpregnataed, and dried again. The impregnated insulators were then fired to Cone 31 to cause vitrification of the ceramic. After such firing, examination of electron photomicrographs of the spark gap surfaces 16 shows a polycrystalline alumina insulator with minute, discrete bodies of elemental platinum in irregular, isolated patches throughout in erstitial material surrounding the alumina crystals. The platinum existed to a depth of approximaately 0.003 inch, at which depth it ended abruptly, as compared to the result obtained when penetration of the noble metal is not controlled. The surface resistance Of the insulators, measured with a 500 volt megger, was not appreciably affected by the impregnation, being 200+ megohms under ambient conditions as was the resistance of a control insulator which had no chloroplatinic acid impregnation.

The procedures described above can also be repeated using aqueous solutions comprising iridium, ruthenium, osmium, or rhodium, and with substantially the same results. Also, the batch material can be shaped and calcined prior to the wax impregnation and firing, and prior to the addition of the aqueous solution.

It will be apparent that various changes and modifications can be made from the specific details set forth herein without departing from the spirit and scope of the invention as defined in the appended claims.

What I claim is:

1. In a method for producing an electrically insulating ceramic body for a spark plug igniter assembly, which method includes the steps of forming a high alumina, spark plug ceramic batch material vitrifiable upon firing to about Cone 31 into a required shape, including a gap surface along which a spark travels when the insulating body is in use as a part of an igniter assembly, and firing the shape to vitrification, the improvement of adding a hydrophobic material in an amount in the order of two percent by weight to said batch material, firing the shape at an elevated temperature in the order of 1500 F. for approximately three minutes, impregnating the fired shape with an aqueous solution comprising about 0.58 gram per milliliter of platinum, drying the shape, and reimpregnating and drying the shape repeatedly until the desired concentration of platinum is achieved, prior to the firing of the shape to vitrification.

2. In a method for producing an electrically insulating ceramic body for a spark plug igniter assembly, which method includes the steps of forming ceramic batch material into a required shape, including a gap surface along which a spark travels when the insulating body is in use as a part of an igniter assembly, and firing the shape to vitrification, the improvement of adding a hydrophobic material in an amount in the order of two percent by weight to the ceramic batch material, firing the shape at an elevated temperature in the order of 1500 F. for two-and-one-half to four minutes, impregnating the fired shape with an aqueous solution of a noble metal, drying the shape, and reimpregnating and drying the shape repeatedly until the desired concentration of the noble metal is achieved, prior to the firing of the shape to vitrification.

3. In a method for producing an electrically insulating ceramic body for a spark plug igniter assembly, which method includes the steps of forming ceramic batch material into a required shape, including a gap surface along which a spark travels when the insulating body is in use as a part of an igniter assembly, and firing the shape to vitrification, the improvement of adding a hydrophobic material to the ceramic batch material, then heating the shape to an elevated temperature for a sufficient period to remove the hydrophobic material from a portion of the shape near the surface thereof, and impregnating at least the gap surface of the shape with an aqueous solution of a noble metal, prior to the firing of the shape to vitrification.

4. A method according to claim 3 characterized by impregnating at least the gap surface of the shape with an aqueous solution comprising from about 0.3 gram per milliliter to about 0.9 gram per milliliter of platinum.

5. A green, unfired electrically insulating ceramic body for a spark plug igniter assembly, said body having a gap surface along which a spark travels, the interior portion of said body at least in the portion near the gap surface containing a hydrophobic material and an outer portion of said body adjacent the gap surface and contiguous with the portion containing the hydrophobic material having therein a multiplicity of minute, separate bodies of a noble metal compound.

5 6 6. An electrically insulating ceramic body for a spark 2,804,392 8/1957 Schurecht 10646 plug igniter assembly, said body having a gap surface 7 2,917,394 12/1959 Schurecht 106-46 along which a spark travels, a portion of said body near 2,926,275 2/ 1960 "Pras 106-46 the gap surface having therein a multiplicity of minute, separate bodies of a noble metal to a depth from 0.002 5 ALFRED LEAVITT, Primary Examinerinch aPPmXimatelY 0-025 inch- W. F. CYRON, Assistant Examiner.

References Cited UNITED STATES PATENTS 2,122,960 7/1938 Schwartzwalder 264-61 XR 10 US. Cl. X. R. 

1. IN A METHOD FOR PRODUCING AN ELECTRICALLY INSULATING CERAMIC BODY FOR A SPARK PLUG IGNITER ASSEMBLY, WHICH METHOD INCLUDES THE STEPS OF FORMING A HIGH ALUMINA, SPARK PLUG CERAMIC BATCH MATERIAL VITRIFIABLE UPON FIRING TO ABOUT CONE 31 INTO A REQUIRED SHAPE, INCLUDING A GAP SURFACE ALONG WHICH A SPARK TRAVELS WHEN THE INSULATING BODY IS IN USE AS A PART OF AN IGNITER ASSEMBLY, AND FIRING THE SHAPE TO VITRIFICATION, THE IMPROVEMENT OF ADDING A HYDROPHOBIC MATERIAL IN AN AMOUNT IN THE ORDER OF TWO PERCENT BY WEIGHT TO SAID BATCH MATERIAL, FIRING THE SHAPE AT AN ELEVATED TEMPERATURE IN THE ORDER OF 1500* F. FOR APPROXIMATELY THREE MINUTES, IMPREGNATING THE FIRED SHAPE WITH AN AQUEOUS SOLUTION COMPRISING ABOUT 0.58 GRAM PER MILLILITER OF PLATINUM, DRYING THE SHAPE, AND REIMPREGNATING AND DRYING THE SHAPE REPEATEDLY UNTIL THE DESIRED CONCENTRATION OF PLATINUM IS ACHIEVED, PRIOR TO THE FIRING OF THE SHAPE TO VITRIFICATION. 